The present invention relates to a transducer, particularly for high pressure measurements.
The measurement of pressures up to 10.sup.9 Pa is a specialist field which is of particular importance in ballistics. Pressures of up to 2.times.10.sup.8 Pa also occur during injection processes in diesel engines. In more recently developed pulse . cutting processes, hydraulic pressure peaks of up to 4.times.10.sup.8 Pa are employed. In practice, however, pressures higher than this will be found only in the development of gun powder and ballistics.
The invention relates in particular, but not exclusively, to transducers for determining such high pressures. A preferred area of application is therefore ballistic pressure measurement in the development of guns and munitions, in order to determine the course of the pressure while a shot is being fired. In powder development, corresponding measurements are carried out in so-called pressure bombs. Another area of application is high pressure measurement in liquid media. In all cases these are processes which last just a few milliseconds and can lead to pressure amplitudes of up to 10.sup.9 Pa.
Copper crusher elements which are still used, particularly in the inspection of munitions, are well known for such measurements. Today, however, electronic measurements by means of piezoelectric transducers in particular are preferred. For dynamic high pressure measurements, piezo-measuring techniques are extremely advantageous because the extraordinarily high resolution enables the course of the pressure to be monitored very accurately from ignition, at pressures of a few Pascals, to combustion at pressure of up to 10.sup.9 Pa. In addition, the piezo-effect as a volume effect in piezocrystals allows a measurement which takes place practically without deformation. The diaphragm sections of such piezotransducers are therefore in principle only subjected to minimum deformations, which essentially assures a long service life.
For the pressure-tight securing of such transducers in a component, e.g. in a pressure vessel, the dimensions of the assembly holes to be made therein, as well as the transducer dimensions, have been largely standardized; NATO in particular has issued standards and regulations to cover this. Two types of sealing devices are used, namely the shoulder seal and the pocket- or blind-hole seal. In Europe transducers with shoulder seal are mainly used, on the basis of relevant NATO regulations, while in the USA the pocket- or blind-hole seal is preferred.
Because of the short-lived pressure thrusts, with unusually high amplitudes, the sealing of the transducer in the assembly hole is extremely important, since the slightest leak can lead to flashovers, as a result of which the transducers may be burnt out and rendered unusable after a few applications. Lapped surfaces are frequently provided as sealing surfaces which, although they do not require additional sealants in the form of sealing rings or the like, suffer from the disadvantage that relapping is required whenever the transducer is removed.
Moreover, thin copper rings have already been used, but again these are difficult to remove from the contact surfaces. Self-adapting steel sealing rings, according to DE-C-17 75 646, have proved particularly advantageous in providing a perfect seal and ensuring a reduction in the torques to be applied for a secure, tight fit of the transducers. However, the tightening torques are comparatively high, and may attain values as high as 60 Nm, which may result in corresponding deformations of the sealing surfaces and other sensitive transducer components, which in turn will affect the measuring sensitivity. Because of the high tightening torques required, no transducers have so far become available whose sensitivity is not influenced in practice by the assembly process.
FIG. 1, which will be discussed in greater detail hereinafter, shows a typical known piezoelectric high pressure transducer with a shoulder seal. The effect of the assembly process on the sensitivity of the transducer stems from the fact that the sealing forces penetrate the sensor section by way of the sealing flange. This gives rise to split spring effects in the area of the plane of separation and deformations of the sensor section, which in turn influence the diaphragm section of the transducer acting on the sensor elements. This leads in particular to a situation where a differnt measuring sensitivity is obtained whenever the transducer is inserted into the assembly hole, rendering it practically impossible to obtain accurate, reproducible measurements. Furthermore, a transducer is known from EU-A-0 090 872 with a shoulder seal, where the sensor member rests, with one of its flat end faces, on an adjacent flat surface of an assembly body recess partially receiving the sensor member, and is welded thereat. An annular gap which is open to the pressure medium to be studied is provided between the recess of the assembly body and the sensor member. Although the annular gap prevents the transfer of the force lines emanating from the shoulder sealing faces to the sensor member, the pressure medium to be examined may reach the inside of the transducer through the annular gap, causing damage.